Amplifier gain control circuit



July 29, 1941.

H. WEBER AMPLIFIER GAIN CONTROL CIRCUIT I Filed July 28. 1938' 2 Sheets-Sheet 1 A 70/ T l T f i i g I /0--# G 1' i l F5 2! a l l l 0 I I I g '1 5 i I I 0.1 L L----i i l I 1 1 0.17, 17, 100% INPUT mm! E I 4 E "2 a Q R 0 gm- 0 5 2 0 B U f $1 INPUT VOLTAGE INVENTQR HEINRICH WEBER ATTORNEYS July 29, 1941. H. WEBER 2,250,559

" AMPLIFIER GAIN CONTROL CIRCUIT Filed July 2a, 1938 2 Shee ts-Sheet g 501/2650, mam/0 pom/rm m 55 H AMPLIFIED oz REL/1 r50 9 Alva c0-m0u0 3 v K 1 k 3"; X aurpu'r i: 5 2 1 I 7/ 6 6 n ,1 3 3 Igl l 2 2 2 fi zw z INVENTOR HEM/RICH WEBER BY gnaw/ ATTORNEYS Patented July 29, 1941 AMPLIFIER GAIN CONTROL CIRCUIT Heinrich Weber, Berlin, Germany, assignor to Telefunken Gesellschait iiir Drahtlose 'l'elegraphie m. b. 11., Berlin, Germany, a corporation of Germany Application July 28, 1938, Serial No. 221,750

In Germany July 23, 1937 4 Claims.

The object of the invention is to provide trans mitter equipment which comprises means designed to insure compensation of fluctuations oi the acoustic intensity or signal strength auto-- matically (AVC) In electro-acoustic transducers or transmitter equipment one important factor is that the acoustic'impulses rendered by the loudspeaker should always be more or less of the same sound intensity, for this insures a maximum of intelligibility. In other words, re-

gardless oi the particular intensity' or sound volume which is brought to act upon the microtained by providing automatic regulators of the,

transmission measure or gain of a kind well known in the prior art which are interposed between the pick-up ,microphone and the loudspeaker, the said volume control means being so designed that equalization of acoustic volume variations reaching the microphone is insured to a point so that the sound volume rendered by the corresponding or associated loudspeaker will give the impression of being practically constant. This tends tov secure an appreciable improvement in the conditions of intelligibility.

Moreover, dit-' only when the signal or useful volume has reached a certain value, such gain increase to extend up to a certain amplitude whereaiter the gain is allowed to diminish again to normal. As a result the output amplitude will be sensed by the ear as being stable between the said two points.

It is known in the prior art that in longdistance telephone iines repeaters designed to maintain a constant sound level are provided for I stabilization of volume. Changes in level as a result of changes-in attenuation on the lines due to temperature etiects, hygroscopic influences, variations in the'working voltages 'oi the intermediate or through-line repeaters, or the like are thus equalized. However, in such schemes the. relationship between the low and the high sound volumes known as the volumeor dynamic ratio is as a general rule maintained. In other words, stabilizing means oithe said sort do not act in the sense or a dynamic compression or of sound-volume equalization. In fact, it is for this reason that these leveling devices are usually controlled by a pilot irequency, which is also sent out over the line, in order to cause the variations of the pilot or control frequency to behave just like the variations in voice frequencies which are occasioned by changes in the conditions of attenuation of longlines.

To carry the basic idea oi this invention in practice, recourse could be had to regulator means designed to act upon the transmission measure Practical experiments have demonstrated that it will be sufllcient to the said end if sound volume variations at the microphone at the ratio of 1:100 are reduced to variations of a ratio of 1:15 to 1:10 in the energy supplied to the loudspeaker.

equipment of the kind here concerned inherent noises as well as such noises as are always present at the microphone during vocal intermissions fbecome audible to a magnified degree, for the reason that the gain which is a function of the average transmission level, is greatly raised,

"whenever thesignal strength level is low. and

this is particularly true during intermissions.

This difliculty, according to another object of 40 'Now, it has been ascertained that also in ulatlng means or this kind have a characteristic such that the requisite volume balance will be insured. In other words, these devices are quasidynamic regulators by which the ratio is com-- pressed to an appreciable degree.

Now, the object of the invention shall be ex.- plained in more detail by reierence to the graphs of Figures 1 and 2, and to the circuit diagram of Figure 3 wherein isillustrated an embodiment of my controlled amplifier system.

Figure 1 shows the inter-dependence of the input voltage E1 and the output voltage E: in

terms of percent, plotted to a double logarithmic scale. Values of are to denote the maximum permissible input and output potential. Graph at shows the operation of the standard amplifier, and graph or the fundamental opera-.

tential is 100% also the output potential should be 100%, This particular point of the charac- 1 are also indicated inthis figure.

teristic is designated by C. The shape of the graphs shows that the gain increases too much in the direction of low amplitudes. For instance, when the input potential drops to 0.1% which means a reduction to one-thousandth of the maximum input potential, the output potential still amounts to 5 percent of the maximum output potential. In other words, it has dropped to only one-twentieth. Hence, the amplification factor at this point is 50 times higher than for point C. Consequently, when using a regulator or control device which operates on the graph A-.C, the noise present during intermissions will be markedly amplified.

According to another object of this invention the regulator or control curve is made to follow the curve DBC, Figure 1. Thus, in the presence of very low amplitudes the control device works like a standard or plain amplifier (straight line portion at an angle of 45) That is to say, there exists a substantially linear relationship between the average values of input and output voltages of the amplifier. As soon as a certain signal amplitude has been reached, say, point D of curve on or as, a pronounced rise in the amplifier gain occurs which extends -as far as point B. Thereafter the gain falls off again to the normal value at C. So far as aural perception is concerned. the output amplitude is stable and constant from B to C.

Figure 2 illustrates the amplification factor as a function of the input potential E1 plotted in terms of percentage. Points AB-C which correspond to the points similarly denoted in Figure The gain is plotted to an arbitrary scale, unity value =1 being present at maximum working level corresponding to point -C. Curves a1 and a'1 illustrate the shape and variation of the amplification factor or gain for difierent control characteristics which, basically, correspond to the-form of curve m in Figure 1. Curves az and a: show which could consistalso of the inner resistance of the transformer and/or the rectifier G1. signal frequency impulses are fed to the rectifier Ga which after rectification charge the condenser Cl. Between point I which corresponds to the positive potential of the condenser C1 and the regulator grid 93 is interposed a battery B: which impresses upon the said regulator grid a negative biasing voltage such that the desired low gain will be present. As long as the input alternating potentials are low. (point D, Figures 1 and 2), the amplifier operates as a straight amplifier with usual gain and they potential is taken off at the output circuit. In other words, up to point D in Fig. 1 there exists a substantially linear relation between the average values of input and output voltages. When the voltages grow larger, i. e., exceed 1%, the condenser C1 is charged up by way of the rectifierGa. This charge opposes source B3 and the grid Go becomes less negative to increase the gain of the tube H. But the charge cannot be higher than the level of the potential of the battery B1, which, with interposition of a rectifier,'say, an oxide-type rectifier G1 isshunted across the condenser C1. The battery potential of Bi corresponds about to the voltage of B3. Inasmuch as the negative biasing voltage 133 of the regulator grid is then compensated by the voltage which arises across the condenser C1, this means that then the highest gain is prevailing in the regulated tube. (Point B, Figure 1.) As the voltage rises still further, as will thus be seen, the potentials taken off at Tzin the output circuit are no longer able to contribute to a change in the regulator or biasing voltage.

' In order to obtain curve shape BC another regulator potential is required. This potential the corresponding, though dissimilar, control characteristics according to the' fundamental curve form an, Figure 1. It will be seen from graphs a1 and ai that for low volume levels there exists a considerable gain, whereas in the case of curves a: and aa, for values less than 1% the gain drops with extremely great rapidity.-

Hence, from D to B the gain rises at a definite rate. From BC, this increment in gain is eliminated.

A graph AC, Figure 1 may be readily obtained with conventional circuit organizations suitably modified for acting upon the gain. The origination of curve D-C, Figure 1 shall now be described by reference to a circuit organization as shown in Figure 3. From Fig. 2 it will'be seen that for input voltages above a predetermined low amplitude there occurs an increase in amplifier gain at a relatively rapid rate, whereas the gain decreases at a relatively slower rate for input voltages whose amplitude substantially exceed the aforesaid predetermined amplitude.

In the exemplified embodiment here illustrated the controlled amplifier tube consists of a hexode H, though it will be understood that the basic idea of the invention may be used also in combination with other tubes, and that also other types of circuits such as push-pull schemes may be employed. A varying potential to be amplifled is supplied from any source to the control grid g1 and cathode of tube H. From the output circuit or the amplifier tube by way of the transformer T2 and the charging resistance R3,

may be derived fromthe alternating input voltage through transformer T1, the charging resistance R4 (which may consist of the inner resistance of the transformer and/or of the rectifier G2), and the rectifier G2. These elements operate to produce the additional potential across condenser C2. The battery B: included in this circuit is such that the rectifier G2 becomes operative only from a certain value of the alternating input potential to charge C: and oppose the drop across 01. Consequently, in the presence of alternating input voltage above the said criticalvalue, the negative biasing potential impressed upon grid as by B: is increased and the amplification of tube H is decreased. Thusrwith increase of the alternating ihput voltage the desired curve shape D-BC, shown in Fig. 1, is secured The rectifier G3 follows a square law relation between input and output for relatively weak signals, as is well known to those skilled in the art. In other words, up to the relatively low amplitude point D in Fig. 1 nobias voltage will be developed across R1 by Ga. Hence, bias from B: will maintain the amplifier gain reduced to such a point that a substantially linear relation exists between input and output voltages.

In the exemplified embodiment here shown the circuit arrangement is so made that, in the presence of low amplitudes, amplified energy is taken 1. In a wave transmission system, an amplifier having a wave input circuit-and a wave output circuit, a source of wave voltage whose voltage amplitude is adapted to extend over a wide range of values; means coupling input circuit, said amplifier havingits constants chosen to provide a linear relation between the average values of input and output voltages over a minimum amplitude portion of said range, means, responsive to amplified wave voltage in said source to said said output circuit, .for controlling the gain-of said amplifier in an increasing'sense upon said source voltage value exceeding said minimum amplitude, and additional-means for controlling a the amplifier gain in a ages whose values are in decreasing sense for voltthe maximum amplitude portion of said range, said amplifier comprising a tube having input and output electrodes. connected to said input and output circuits respectively, said tube including a special gain control electrode, and each of said just the potential thereof. 4

2. In a wave transmission system, an amplifier having a wave input circuit and a wave outputcircuit, a source of .wave voltage whose voltage amplitude is adapted to extend over a-wide range of values, means coupling said source to said input circuit, said amplifier having its constants chosen toprovide a linear relation between the average 'values 01 input and output voltages over a minimum amplitude portion of said range, means, responsive to amplified wave voltage in said output circuit, for controlling the gain of said amplifier in an increasing sense upon said source voltage value. exceeding said minimum amplitude, and an auxiliary control network, responsive to the input circuit voltage, for decreasing the gain of the amplifier upon the source voltage exceeding a predetermined intermediate value of said range, said amplifier comprising a tube having input and output electrodes connected to said input and output circuits respectively. said tube thereof.

including a special gain control; electrode, and each of said gain control means being connected to said special electrode to adjust the potential 3. In combination with a signal amplifierhaving input and output circuits, means responsive to signals of an intensity in excess of a predetermined amplitude for increasing the gain of 'the amplifier at a relatively fast rate, means to prevent operation of said, responsive means for signals having an intensity of a predetermined amount higher than said predetermined ampli-' tude,-and an auxiliary control means responsive to signals whose intensity exceed an amplitude ing input and output circuits,

ain control means being connected to said special electrode to adsignals having an intensity of a substantially higher than said predetermined amplitude, for decreasing the amplifier gain at a substantially slower rate than said first rate.

4. In combination with a signal amplifier havto signals of an intensity in excess of-a predetermined amplitude for increasing the gain of the amplifier ata relatively fast rate, means to prevent operation of said responsive means for predetermined amount higher than said predetermined amplisaid input circuit.

tude, and an'auxiliary control means, responsive to signals whose intensity exceed an amplitude substantially higher than said predetermined amplitude, ,for decreasing, the amplifier gain at a substantially slower rate than said first means being connected circuit, said auxiliary means HEINRICH WEBER.

means responsive said first rate," to said ou put ;being connected to. 

